A hub bearing (HUB) is a bearing device for a wheel for rotatably supporting a tire (wheel) on a vehicle body. Also, a Constant Velocity Joint (CVJ) is a torque transmission unit for transmitting the rotation from an engine to a wheel. The HUB and the Constant Velocity Joint have the following manufacture problems:    (1) A stepped shape requiring a high working ratio is included.    (2) A presence of rolling contact surfaces requires portions with high hardness after high-frequency induction hardening process. For this reason, steel having a high carbon content such as S53C or SAE1050 is used, thereby increasing deformation resistance in the working process.
Due to the situation described above, cold forging and warm forging are difficult and therefore hot forging is widely used. In the case of hot forging, the forging temperature reaches as high as 1000° C. or more, and therefore the micro structure after forging is coarsened.
Generally, in the bearing device for a wheel, a large load is repeatedly imposed, and therefore the hardening process is carried out after the hot forging process thereby to secure a sufficient strength of the bearing device for a wheel. In this bearing device for a wheel, a roll member is held from both sides by an outer member (outer ring) and an inner member (inner ring). The inner member is configured of two first and second inner members such as a HUB ring and an inner ring. To promote the reduction of the number of parts, the caulking process is used as a coupling means for coupling the two inner members.
In the case where the hardening process is executed after the hot forging as described above, the hardness of the product extremely increases and therefore the caulking process is hampered. For this reason, a roll bearing unit for supporting a wheel has been proposed (Japanese Patent Laying-Open No. 11-129703 (Patent Document 1)) in which the portion plastically deformed by the caulking process is not hardened in the hardening process. In this method, the portion plastically deformed by the caulking process is not hardened, and therefore the hardness of the particular portion is reduced.
Patent Document 1: Japanese Patent Laying-Open No. 11-129703